A standard single- or multistage panel press used for instance to manufacture plywood, particleboard, or flakeboard has a housing formed by a plurality of annular and rectangular tension frames lying in respective horizontally spaced vertical planes. Each tension frame has a horizontal lower element or beam, a horizontal upper element or beam, and a pair of vertical and horizontally spaced side elements each having an upper end secured to the respective upper beam and a lower end secured to the respective lower beam so as to form a window. The frames support a pair of vertically spaced horizontal platens one of which can be moved vertically toward the other, typically by a heavy-duty hydraulic actuator. One or more workpiece panels carried on respective plates or belts are positioned in the plurality of frames between the platens and the one platen is urged toward the other to compress the workpiece or workpieces, so as to compact them and cure a binder in them.
In U.S. Pat. No. 5,611,271 of F. Bielfeldt a window-type tension frame of the above-described type is described for a multistage press. Each of the vertical frame elements is secured to the upper and lower beams by complex joints held together by vertical tie screws. Special brackets are provided to anchor these screws, so that construction is complex and expensive. Furthermore servicing such a press is difficult in that the steps to disassemble and reassembly a single window-type frame are quite extensive.
Another system described in German patent 19,500,983 published 21 Dec. 1995 uses a simpler brute-force approach in that the side elements and beams have lateral flanges that are bolted together. Thus these attachment bolts carry all the load. Such an arrangement is bulky and, once again, entails complex construction at the locations where the vertical side elements, which are stressed virtually only in tension, are connected to the upper and lower beams.